The most important effects of dental restorative materials are considered to be those which directly impinge on the physiology of the human host. However, our recent laboratory studies have demonstrated that installation of amalgam fillings in monkeys results in a rapid proliferation of Hg and antibiotic resistant oral and intestinal bacteria. The possibility that this amalgam-provoked increase in bacterial resistance might also occur in humans is suggested by epidemiological data indicating a strong correlation between the occurrences of Hg resistance and of multiple antibiotic resistances in the intestinal flora of humans, even those known not to have recently consumed an antibiotic. Unfortunately, data specifically correlating the proliferation of resistant flora to amalgam status of individual humans are presently lacking. The current public health concern with the increasing prevalence of antibiotic resistant pathogens makes it very important to determine whether the routine exposure to Hg arising from dental amalgams can stimulate the proliferation of multiply antibiotic resistant bacteria in the human host. Thus, we propose to quantify the changes in Hg and antibiotic resistances in the gingival and intestinal bacteria of individual human subjects on the occasion of their receiving their first dental amalgam restorations. We will test the hypothesis, arising from our animal studies, that installation of amalgam fillings in a naive subject (i.e. one who has never had any amalgam fillings), leads to a proliferation of rare pre- existing Hg and antibiotic resistant bacterial strains in both the gingival and intestinal flora. It is important to determine whether human flora responds in the same way as the monkey flora because, such an enrichment of antibiotic resistance in human normal flora in response to the selective pressure of Hg released from dental amalgam would constitute a novel and previously unrecognized risk factor in the widespread use of amalgam dental restorations.